When rock or soil mass of the earth is disturbed, either by natural events or by the activities of man, it undergoes a redistribution of stress, accompanied by a change of shape. The change of shape may produce undesirable effects such as failures of slopes, cave-ins of mine tunnels, damage to other underground structures, and leakage from reservoirs, empoundments, and toxic or nuclear waste repositories. Changes of shape are often considered as deformations or displacements and represent complex effects acting over substantial and significant distances often up to hundreds or even thousands of feet. They are materially different from strain which is most often considered to be an effect acting over some finite distance, usually very short. Strain usually comprises the effect as related to a single structural element as detected between two connections or two discontinuities or the like.
The sources of disturbances are typically associated with the excavation of material as by the removal of lateral support, changes in the operating loads as where structures are subjected to reservoir weights or traffic loads, and the occurrence of heat as, for example, can occur with radioactive waste repositories. Heat, which is a principal source of disturbance in repository operation, is also a secondary factor in other applications to the extent that temperature variations produce low levels of deformation, i.e., change of shape, which cannot always be distinguished from those changes produced by loading or by the removal of lateral support.
The stress redistribution produced by disturbances may not usually be measured directly. The deformation e.g. a change of shape, on the other hand can be measured reliably, using general techniques which have established precedents and wide acceptance.
For the effective measurement of very small amounts of deformation, however, temperature changes become a significant source of error, sometimes to the extent that such errors substantially conceal important and otherwise measurable deformation.
Conventional extensometers of the rod or wire type have been used to measure axial strain in a borehole in rock. They are materially distinguishable from devices that measure effects on structure transverse of the instrument, as in strain gauges. Where the temperature of the rock mass is being raised, some systems suffer from the effects of thermal expansion of the rod or wire. A string of thermocouples are usually installed with the extensometer and regular temperature readings must be taken throughout the life of the extensometer so that the total thermal expansion of the rods can be calculated to separate out any strain component.
In the past, it has been necessary to attempt to correct for the effects of temperature changes by either fabricating deformation measuring equipment of materials with low coefficients of thermal expansion (INVAR, etc.) and assuming them to be unaffected by temperature changes, or by designing the instrument in such a way that the temperature effects on the various components can be calculated, and the deformation data adjusted accordingly. The problems involved in these approaches are as follows: (1), the desirable properties of low temperature coefficient materials are often lost if temperatures exceed certain limits, sometimes limits which are well within potential operating ranges; (2), calculated corrections require that accurate temperature data be available over the entire physical extent of the measuring device or devices, a costly, frequently impractical, and sometimes imposssible consideration.